Circuit breakers are widely used in industrial, commercial and residential applications for protecting electrical conductors and apparatus from damage due to excessive current flow. Initially used as a direct replacement for fuses, circuit breakers have been gradually called upon to provide more sophisticated types of protection other than merely interrupting the circuit when the current flow exceeds a certain level. More elaborate time-current trip characteristics have been developed such that a circuit breaker can rapidly open upon very high current with the time delay being roughly inversely proportional to the degree of overload. Circuit breakers are also available which interrupt upon the detection of ground fault currents. As the complexity of electrical distribution circuits has increased, the control portions of the circuit breaker have been interconnected to provide selected coordination.
During the late 1960's, solid state electronic trip units were developed for use in high power, low voltage circuit breakers. These electronic trip units performed functions such as instantaneous and delayed tripping which were traditionally achieved by magnetic and thermal means. The improved accuracy and flexibility of the solid state electronic trip units resulted in their wide spread acceptance.
The earliest electronic trip unit designs utilized discreet components such as transistors, resistors and capacitors. More recently, designs such as disclosed in U.S. Pat. No. 4,428,022 have included microprocessors which provide improved performance and flexibility. Due to the severe space limitations in low voltage circuit breakers, the assignee of this application has developed a special purpose integrated circuit known as a SuRE Chip.TM., which incorporates a microcontroller core processor, volatile and nonvolatile memory, and an eight bit analog-to-digital converter, four bit pre-ranging circuit amplifiers, comparators, and an input analog multiplexer which provides all of the essential analog and digital circuit functions in a single monolithic device. This device is described in detail in U.S. Pat. No. 5,525,985, issued Jun. 11, 1996.
These digital systems sample the current waveforms periodically to generate a digital representation of the current. The microprocessor uses the samples to execute algorithms which implement the protection curve which is typically based upon a constant value of I.sup.2 t where "I" is the value of current and "t" is the time-to-trip. Typically in air circuit breakers and vacuum interrupters, the protection curve provides an instantaneous trip, a long delay trip and, if appropriate, a short delay trip function. In some circuit interrupters, the microprocessor also performs calculations for metering such as determining the RMS value of the highest phase current.
It is also common today to have a plurality of such circuit interrupters monitored and controlled by a central network control station such as is described in U.S. Pat. No. 5,420,799, issued May 30, 1995. In turn, the circuit interrupters usually have mechanical status indicating accessory devices mounted within the casings which are used by external and remote monitoring and control equipment. Examples of these are auxiliary contacts which follow the open/close status of the circuit breaker and bell alarm contacts which are closed if the breaker is in the tripped condition. While the central monitoring and control network typically communicates digitally with multiple circuit interrupters over a common network, the connection between the internally mounted mechanical contacts within the individual circuit interrupters and the remote slave devices is accomplished in parallel using two wires per pair of contacts. Usually multiple sets of auxiliary contacts are required and thus multiple pairs of wires are needed. The installation of this wire is costly and time consuming. In addition to digital status information, it is also desirable to provide certain analog information such as the value of load current, bus voltage, power factor, etc. Many remote utilization devices, such as programmable logic controllers, require this information as 4-to-20 ma current values. Typically, circuit breaker trip units don't provide 4-to-20 ma outputs.
The increase in related functional versatility that the electronic trip units of circuit breakers have added employing the capabilities of microprocessors has required the processing circuitry to monitor wide ranges in current and related parameters which can vary from open or a zero state, to a normal operating range where most of the metering calculations are performed, to the other extreme or short circuit states which exceed the normal operating range by a factor 10 or greater. Scaling of the monitored value is required to enable the microprocessor to monitor the entire range below its saturation limitations. The extent of scaling required to accommodate the trip values can affect the accuracy of the calculations performed in the metering range, the normal operating state of the protected device. It can also affect the microprocessor's ability to monitor harmonics and small distortions in the current.
Accordingly, an improved circuit interrupter is desired that has more flexibility in communicating with its auxiliary slave devices and has increased sensitivity to the monitored current over its normal operating range.